<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>Insert title here</title>
<style type="text/css">
	body>div{
		width: 200px;
		height: 200px;
		margin: 30px 0 0 30px;
		background-color: red;
		overflow: hidden;
		position: relative;           /*做位置参考*/
	}
	body>div>div{
		width: 100px;
		height: 100px;
		margin:30px 0 0 30px;
		background-color: green;
	
	}
	div>div>div{
		width: 50px;
		height: 50px;
		background-color:blue;
		/* margin:30px 0 0 30px;  */
		position: absolute;             /*绝对定位*/
		left:0;
		top:0;
	}
</style>
</head>
<body>
	<div>这是div1
		<div>这是div2
			<div>这是div3
			</div>
		</div>
	</div>
</body>
</html>